In recent years, polyester films have been frequently used as various kinds of optical films and molding films which are employed in various applications such as touch panels, antireflection films, prism sheets, light diffusion sheets and electromagnetic shielding films as members of liquid crystal displays or plasma displays as well as in-mold transfer films and in-mold label films. A base film used for these members has been required to have excellent transparency and visibility.
These films have been often subjected to hard coating treatment in order to enhance an anti-curling property, a mar resistance and a surface hardness thereof, etc. Also, as a base material for the films, there have been generally used polyester films which are excellent in transparency and mechanical properties. In order to enhance adhesion between a polyester film and a hard coat layer, a coating layer having an easy-bonding property is generally provided as an intermediate layer therebetween. For this reason, refractive indices of these three layers including the polyester film, the easy-bonding coating layer and the hard coat layer must be taken into consideration to avoid occurrence of interference fringes.
When using the films having interference fringes in displays such as touch panels, the displays tend to exhibit a poor visibility, resulting in difficulty in handling thereof upon use. For this reason, it has been required to take a suitable measure against the interference fringes. In general, it is considered that the refractive index of the coating layer at which occurrence of interference fringes can be reduced is present near a geometrical mean value of a refractive index of the polyester film as the base material and a refractive index of the hard coat layer. Therefore, it is ideally required to adjust the refractive index of the coating layer near to the geometrical mean value. In this case, since the polyester film has a high refractive index, it has been generally required that the coating layer is designed to have a high refractive index.
As the method of enhancing a refractive index of the coating layer to prevent occurrence of interference fringes, there is known, for example, the method in which a metal chelate compound is used in combination with a resin to enhance a refractive index of the coating layer (Patent Document 1). However, in this method, owing to unstableness of the metal chelate compound in an aqueous solution, a coating solution used therein also tends to become unstable according to combination between the compound and resin, so that the procedure of replacing the coating solution with new one must be frequently repeated when continuing the film production process for a long period of time. In addition, the coating layer tends to be insufficient in wet heat resistance owing to use of the metal chelate. Therefore, it has been required to provide a coating layer which is excellent in stability of a coating solution thereof as well as wet heat resistance.